Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates generally to digital wireless telecommunications systems. More particularly, the invention relates to a system and method for warning of, and providing greater immunity from, an impending call drop in a digital cellular telephone system, personal communication service (PCS) network, or equivalent enterprise employing variable frame rate data transmission.
2. Description of the Prior Art
In analog wireless service networks, such as Advanced Mobile Phone Service (AMPS) systems, a mobile subscriber can anticipate that a call may be dropped (abruptly terminated while conversing) if the subscriber hears static superimposed upon speech. This can occur when the mobile unit traverses the boundary of RF coverage.
In digital wireless telecommunications systems, such as CDMA and TDMA cellular telephone networks, the subscriber does not experience impending call drop static. Up until the instant the call is dropped, the voice quality is clear, and then the call terminates abruptly. This unexpected dropping is not desirable.
Various prior art proposals have been made to generate an audible tone that serves as an impending call drop warning. Rather than using annoying tone signals, however, it would be preferable if a more subtle call drop warning could be provided to the subscriber. It would be further advantageous if this warning could be implemented with a technique for delaying or even avoiding the impending call drop so as to provide increased call drop immunity.
The foregoing problem is solved by the system and method of the present invention, which provides a warning of, and greater immunity from, an impending call drop in a digital wireless telecommunications system. During normal operations, data is transmitted in the telecommunications system at an initial information frame rate and a corresponding initial physical frame rate. Upon the data transmission producing errors in excess of a threshold, which is indicative of an impending call drop, the initial information frame rate is reduced while error correction codes are introduced into the data transmission to maintain the initial physical frame rate. The reduction in the information frame rate produces a subtle yet noticeable change in acoustic voice quality, which serves as a warning of the impending call drop. The introduction of error correction codes into the transmission provides greater immunity to the impending call drop by improving data integrity.
Typically, the wireless telecommunications system includes a Mobile Switching Center (MSC) coupled to at least one radio base station servicing at least one mobile radio unit. The base station and the mobile unit communicate with each other in a digital transmission mode wherein voice data is digitally encoded by voice encoder/decoders (vocoders) located in the MSC (or the base station) and the mobile unit as a sequence of variable rate information frames. The information frames are packaged by physical layers (which may be implemented as part of the vocoders) into a sequence of physical transmission frames that include the information frames with appended and/or prepended control and error checking bits.
In preferred embodiments of the invention, the telecommunications system is programmed to monitor normal transmissions between the base station and the mobile unit for frame errors. During such normal transmissions, the vocoders encode voice data at an initial information frame rate and the physical layers receive the initial rate information frames from the voice encoders and construct physical transmission frames at an initial physical frame rate corresponding to the initial information frame rate.
When a call drop is impending, the number of frame errors increases. When it reaches a predetermined threshold, the telecommunications system controls the vocoders to reduce the information frame rate from the initial information frame rate to a reduced information frame rate, such that reduced rate information frames are provided to the physical layers. The telecommunications system then controls the physical layers to add error correction bits to the reduced rate information frames to transmit error correction enhanced physical transmission frames at the initial physical frame rate.
The error correction enhanced physical transmission frames produce a characteristic change in speech quality due to the reduced information frame rate. This change in speech quality provides a subtle but effective call drop warning. Preferably, the reduced rate information frame carries no more than half the information of the initial information frame.
The error correction enhanced physical transmission frames also provide increased error correction robustness for greater immunity from the impending call drop. To that end, the difference between the initial information frame rate and the reduced information frame rate is selected to accommodate sufficient error correction bits to achieve a predetermined number of correctable errors per frame. Preferably, these additional error correcting bits constitute a cyclic error control code, and most preferably, a shortened BCH cyclic code. The error control code is selected so as to correct a predetermined number of correctable errors in a frame that is the same size as the initial information frame.
By way of example, assume the initial information frame has n information bits and the reduced information frame is selected to have k information bits. The error correction enhanced physical transmission frame will be capable of carrying nxe2x88x92k additional error correction bits representing a shortened cyclic error control code capable of correcting t errors in a frame of size nxe2x80x2xe2x89xa7n and kxe2x80x2xe2x89xa7k, where nxe2x80x2 is the correctable frame size and kxe2x80x2 is the number of correctable frame information bits 7. In the event fewer than nxe2x88x92k additional error correction bits are required to correct t errors in a frame of size nxe2x80x2xe2x89xa7n and kxe2x80x2xe2x89xa7k, the error correction enhanced physical transmission frame is constructed to include additional place holder bits.
The invention may be implemented without returning to the normal transmission mode until a call is terminated. Alternatively, the initial information frame rate could be resumed in the event the number of frame errors falls below the predetermined threshold.